Top Most the java.lang Package Interview questions

1. Wrappper classes :-

·         Wrapper classes are immutable.

·         given,

Float f  = new Float(“Str”);

Str can take values	Str can’t take values
123 or +123 i.e. positive integer(with ‘+’ sign)	1l or 1L i.e. suffixing ‘L’ will cause error.
-2343 i.e. negative integer	0x10 i.e. we cannot give hexadecimal numbers as input.
±1.0 or ±.0d i.e. double number
±.0f i.e. float number
±1.1e5f i.e. we can have ‘e’ in the input string
±1.1e5d i.e. we can have ‘e’ in the input string

·         given ,

Long l = new Long(“Str”);

Str can take values	Str can’t take values
123  i.e. positive integer without ‘+’ sign	+123 i.e. positive number with ‘+’ sign
-234 i.e. negative integer	0x133 i.e. hexadecimal number
	±1.0f i.e. floating point number
	±1.0 i.e. double precision number
	±.1e1f or ±.1e2d i.e. ‘e’ is not allowed
	1l or 1L i.e. suffixing ‘L’ is not allowed

·         Float constructor is overloaded,

Ø  takes a primitive “double number”

Ø  takes a string

Ø  takes a primitive “float number”

·         The equals method when used with wrapper classes gives output as true if

Ø  both input objects are of the same class

Ø  both have the same value

→ Remember if two objects of different class are input to equals method then the output is false and not compiler or runtime error.

·         there isn’t a no argument constructor in any of the wrapper classes.

·         given,

Byte b = new Byte(1);//causes compiler error because 1 → is an integer and Byte constructor takes only byte .similarly for short.

·         given,

Long lg1 = new Long(1);

Long lg2 = new Long(lg1);//causes compiler error because Long constructor doesn’t take in Long object

·         <wrapperClassName>.toString(); is overloaded

Ø  takes a no argument constructor and returns string of the given wrpper object

Ø  (static)takes argument corresponding to the wrapper class i.e.

Integer.toString()  takes int number and Float.toString() takes float number only unlike its constructor

·         wrapper classes ( excluding Character and Boolean) extends Number class and has 6 methods and they are

Ø  byteValue

Ø  shortValue

Ø  intValue

Ø  longValue

Ø  floatValue

Ø  doubleValue

→ there isn’t a “charValue” int the wrapper class(excluding Character).

·         Boolean.valueOf() returns a reference to either Boolean.TRUE or Boolean.FALSE

Boolean b1 = Boolean.valueOf(true);

Boolean b2 = Boolean.valueOf(“TrUe”);

 “b1==b2“ is true.

·         Remember NaN is not equal to Nan.

·         Boolean has two constructors,

Ø  takes boolean values i.e. true or false.

Ø  takes String values i.e given,

Boolean b = new Boolean(“TrUe”);//input string if passes the equalsIgnoreCase method with the string “true” then the new Boolean object will contain value - true.

Ø  given,

Boolean b = new Boolean(<<AnyStringLiteral>>);

for e.g. Boolean b = new Boolean(“santosh”);

            Boolean b1 = new Boolean(null);

            then , b.equals(b1) will be true because b and b1 contain – false .

·         given,

Double a = new Double(Double.NaN);

Double b = new Double(Double.NaN);

then a.equals(b) will be true.

·         Given,

Double c = new Double(0.0);

Double d = new Double(-0.0);

c.equals(d) → gives the output of false.

·         Remember “0.0 == -0.0” is true.